US Secretary of State John Kerry, sitting left, and Russian Foreign Minister Sergey Lavrov, sitting right, wait to start their meeting at Winfield House, the residence of the US ambassador, in London, Friday March 14, 2014. U.S. Secretary of State John Kerry flew to London on Friday to meet with Russian Foreign Minister Sergey Lavrov in a last-minute bid to stave off a new chapter in the East-West crisis over Ukraine. AP

US Secretary of State John Kerry, sitting left, and Russian Foreign Minister Sergey Lavrov, sitting right, wait to start their meeting at Winfield House, the residence of the US ambassador, in London, Friday March 14, 2014. U.S. Secretary of State John Kerry flew to London on Friday to meet with Russian Foreign Minister Sergey Lavrov in a last-minute bid to stave off a new chapter in the East-West crisis over Ukraine. AP

Over the past year, Secretary of State John Kerry and Russian Foreign Minister Sergey Lavrov have worked closely on controversial issues, seemingly partnering on Syrian chemical weapons disarmament, bringing the two sides in the Syrian conflict together in Geneva and working in tandem to persuade Iran to make concessions over its nuclear program.

But on Friday, after hours of conversation at a negotiating table and during sun-splashed walks on a manicured lawn, the two men emerged with a bleak message: They couldn’t reach common ground on the future of Ukraine. There was simply no wiggle room in either’s position to ease the worsening crisis.

Kerry’s starting point was an intact and independent Ukraine; Lavrov’s was a Ukraine without Crimea. Their positions were so far apart that Kerry didn’t even get to fully brief his counterpart on all his deal-sweetening ideas – Lavrov made it clear that the only person who could change Crimea policy was President Vladimir Putin.

Both men looked visibly dejected when the talks ended.

“We were prepared to float them on Ukraine’s behalf – to test market them, if you will, with the Russians – and we started down that path today,” a senior State Department official, speaking on condition of anonymity so as to freely discuss diplomacy, said of a variety of proposals. “But it very quickly became clear that Foreign Minister Lavrov was not empowered to discuss any aspect of any proposal that might have an impact in Crimea.”

The failure of the London talks sets in motion a chain of events that veteran diplomats and analysts fear could inflame Europe: Crimea will vote Sunday to join Russia, the United States and its allies will impose sharper sanctions against Moscow in retaliation, and thus begins a cycle of tit-for-tat maneuvers that could torpedo other joint U.S.-Russian projects and stoke fears among Russia’s neighbors.

Col. Steve Warren, a Pentagon spokesman, said Friday that six U.S. warplanes had arrived Thursday in Poland for a beefed-up U.S. role in NATO military exercises and six more of the F-16 fighter jets would land there Friday.

A C-17 transport plane brought 150 U.S. Air Force and other military personnel to Poland to support the increased flight training exercises with Polish airmen, and a C-130 cargo aircraft carried 25,000 meal rations, Warren said.

The aircraft carrier USS George H.W. Bush, accompanied by two guided-missile destroyers and a guided-missile cruiser, would remain in the Mediterranean Sea “for several days,” Warren said, on what he described as part of a previously scheduled exercise.

Warren added that “anything could happen” to alter the U.S. warships’ itinerary, but for now they were not expected to remain in the Mediterranean beyond the first part of next week.

“A lot of what we’re doing there now is an effort to reassure our allies,” Warren told reporters at the Pentagon.

Kerry’s meeting with Lavrov capped a week that saw the United States and Europe turbocharging diplomacy in hopes of stopping the referendum Sunday, in which Crimea residents will vote on whether to join Russia or become independent; remaining with Ukraine is not an option. Kerry and others in the administration have said plainly that Russia would face sanctions, even as early as Monday, if the vote proceeds as planned.

“There will be consequences if Russia does not find a way to change course,” Kerry told reporters after the meeting with Lavrov. “And we don’t say that as a threat. We say that as a direct consequence of the choices that Russia may or may not choose to make here.”

So far, neither overtures nor threats have swayed Putin. He shows no sign of taking what the Obama administration calls the “offramp” option: sending Russian troops in Crimea back to their barracks and opening direct talks with Ukraine’s provisional government.

Instead, U.S. officials say, Putin is massing thousands of troops along the border in a snap exercise that Western observers fear is a signal of broader incursion, perhaps into eastern Ukraine. That would nudge the conflict into an even more dangerous period, with the prospect of shooting between Russians and Ukrainians and a NATO buildup in Ukraine’s neighbors.

“We also remain deeply concerned about the large deployments of Russian forces in Crimea and along the eastern border with Russia, as well as the continuing provocations and some of the hooliganism of young people who’ve been attracted to cross the border and come into the east,” Kerry said.

Western observers worry that the land grab in Crimea is only the beginning of a campaign by Putin to reconstitute the Soviet Union. Putin, who famously lamented the breakup of the Soviet Union, says he’s merely protecting ethnic Russians in the power vacuum left by the ouster of Ukrainian President Victor Yanukovych, who was close to Moscow.

Putin has refused to open direct talks with Yanukovych’s successor, the Western-friendly interim government that will rule until after presidential elections that are scheduled for May. Interim Ukrainian Prime Minister Arseniy Yatsenyuk was in Washington earlier this week on a high-profile trip that read as a show of U.S.-Ukrainian solidarity in the face of Russian aggression – hardly a reassuring move for Putin, who’s loathe to see a Kiev that isn’t aligned with Moscow, its traditional patron.

However, in remarks at the White House and at the Atlantic Council, Yatsenyuk also offered some conciliatory words for Putin. He said that the interim leadership seeks to preserve ties with Russia, but in a normal bilateral relationship and not an “incursion.”

Yatsenyuk welcomed international monitors to help protect ethnic Russians and other minorities, and he added that the Ukrainian Parliament was open to discussing in a more constitutional way options for Crimea’s future.

Had Putin played his cards right, the Americans suggest, he could’ve had a Crimea that was annexed in all but name. The interim government, they say, was prepared to give Crimea near-total autonomy, but Putin rejected it.

Lavrov and Kerry agreed to remain in touch in the uncertain days ahead.

“Both guys wanted to keep the door open, although we don’t know what’ll be on the other side,” the senior State Department official said.

James Rosen in Washington contributed to this report.

Russian Foreign Minister Sergey Lavrov, left, and US Secretary of State John Kerry stand together before a meeting at Winfield House in London, Friday March 14, 2014. U.S. Secretary of State John Kerry flew to London on Friday to meet with Russian Foreign Minister Sergey Lavrov in a last-minute bid to stave off a new chapter in the East-West crisis over Ukraine. AP

Russian Foreign Minister Sergey Lavrov, center, waits to start the meeting with US Secretary of State John Kerry at Winfield House, the residence of the US ambassador, in London, Friday March 14, 2014. U.S. Secretary of State John Kerry flew to London on Friday to meet with Russian Foreign Minister Sergey Lavrov in a last-minute bid to stave off a new chapter in the East-West crisis over Ukraine. AP